The presence of cholesterol side-chain cleavage enzymes in the rat testis was established by Shimizu, Hayano, Gut & Dorfman (1961). More recently Hall, Irby & DeKretser (1969) separated interstitial cells from seminiferous tubules in an attempt to compare the relative abilities of interstitium and seminiferous tubules to convert cholesterol to testosterone in incubations in vitro. They reported that the interstitial cells represent the major and probably only source of testicular androgens. This conclusion failed to explain the significance of the findings by Bell, Vinson & Lacy (1968, 1971) that the seminiferous tubules can convert pregnenolone to androgens. Therefore, the cholesterol side-chain cleavage activity of both the interstitium and seminiferous tubules of rat testis was examined by extracting and measuring the labelled isocaproic acid produced by the tissues when incubated with [26-¹⁴C]cholesterol.

Mature male Wistar rats (body weights 240–360 g) were killed with coal gas. The testes were removed

Plasma samples from intact, adrenalectomized, adrenalectomized and castrated and castrated bulls were assayed for LH, testosterone, androstenedione and oestradiol-17β from birth to 26 weeks of age. The adrenalectomized bulls, unlike the intact bulls, failed to show a rise in androstenedione at 14·5 weeks of age or a rise in testosterone at 20 weeks of age. Testosterone levels in the castrated animals remained below 0·4 ng/ml whereas androstenedione reached levels similar to those in intact bulls by 26 weeks of age. In all animals the concentration of oestradiol-17β in plasma remained below 25 pg/ml, although intact bulls had the highest levels. Levels of LH rose after castration but not after adrenalectomy. These data show that in bull calves absence of the adrenal glands during prepuberty delays the rise in pubertal testosterone by at least 10 weeks.

Ministry of Agriculture and Fisheries, Research Division, Ruakura Agricultural Research Centre, Hamilton, New Zealand

(Received 17 April 1978)

An increase in the plasma concentration of luteinizing hormone (LH) occurs in response to castration in bull calves aged 1–4 months; this response is of similar magnitude to that seen in cattle castrated as adults (Odell, Hescox & Kiddy, 1970). In bull calves castrated at birth, however, there is no increase in the plasma concentration of LH until after 28 days of age (Bass, Peterson, Payne & Jarnet, 1977). In other species a range of responses to castration has been reported. Gonadectomy of male guinea-pigs 0–35 days after birth produces an increase in the plasma concentration of LH similar to that observed in guineapigs castrated as adults (Donovan, ter Haar, Lockhart, MacKinnon, Mattock & Peddie, 1975). In contrast, the castration of young male macaques does not cause an immediate increase in the

ABSTRACT

The binding of bovine GH (bGH) to hepatic membranes obtained from steers on either high (3% dry matter of body weight per day) or low (1% dry matter of body weight per day) planes of nutrition with or without an oestradiol-17β implant was studied (n = 5 per group). Binding studies were performed on both crude membrane homogenates and on 100 000 g microsomal membrane fractions; identical results were obtained using both preparations. In all four groups of animals, linear Scatchard plots were obtained, but following pretreatment of the membranes with MgCl₂ to remove endogenously bound hormone, curvilinear plots were obtained in the groups on the high plane of nutrition. Analysis of these curves suggested the presence of a high- and low-affinity binding site, the high-affinity site being fully occupied in the absence of MgCl₂ pretreatment.

The specific binding of bGH in MgCl₂-pretreated crude membranes was greater (P < 0·01) in well-fed steers (14·8 ± 1·6%) than in poorly fed steers (9·8 ± 0·9%). Scatchard analysis showed this to be due to the presence of a high-affinity site (dissociation constant (K _d) = 11·6 ± 3·3 pmol/l) in the well-fed animals only. In addition, there was an increase (P < 0·01) in the affinity, but not in the capacity, of the low-affinity site (K _d = 106·4 ± 22·8 pmol/l in well-fed steers and 197·0 ± 23·8 pmol/l in poorly fed steers).

Oestradiol treatment was associated with an increase (P < 0·01) in specific binding at both planes of nutrition, but binding was higher (P < 0·01) in well-fed (24·8 ± 2·9%) than in poorly fed (15·6 ± 3·7%) steers. Scatchard analysis after MgCl₂ pretreatment again showed a curvilinear plot at the high and a linear plot at the low nutritional plane. The effect of oestradiol was to increase (P < 0·001) the capacity of the high-affinity site from 1·87 ± 0·61 pmol/100 mg in the control well-fed group to 6·56 ± 1 ·2 pmol/100 mg. The capacity of the low-affinity site was increased (P < 0·01) from 20·1 ± 2·6 to 30·1 ± 3·2 pmol/100 mg in the well-fed group, with a similar change in the poorly fed group. Oestradiol had no effect on the apparent affinity of either binding site.

These studies demonstrate a heterogeneity of somatotrophic binding sites of hepatic membranes in steers. The presence of a high-affinity site is determined by nutritional status, whereas oestradiol primarily affects receptor capacity. Thus nutrition and oestradiol have independent and qualitatively different effects on somatotrophic binding. As the rate of weight gain correlated (P < 0·01) with the capacity of the high-affinity site, it is suggested that somatotrophic receptor modulation is a primary factor in the regulation of somatic growth in the ruminant.

J. Endocr. (1988) 116, 169–177

ABSTRACT

Plasma GH profiles and circulating concentrations of plasma insulin-like growth factors-I and -II (IGF-I and -II) were examined in 20 steers on either high (3% dry matter of body weight per day) or low (1% dry matter of body weight per day) planes of nutrition with or without an implant of oestradiol-17β. The response of plasma IGF-I and -II to a bolus injection of bovine GH (bGH) was also investigated.

Reduced feeding significantly (P <0·01) increased the mean concentration, peak height and integrated area of plasma GH. Treatment of steers with oestradiol at low nutrition significantly increased baseline GH concentrations. Treatment of steers with oestradiol at high nutrition significantly (P <0·05) increased mean, baseline, peak height, and integrated area of plasma GH. GH pulse frequency was not changed by either nutritional plane or oestradiol treatment.

Basal concentrations of plasma IGF-I were significantly (P <0·01) decreased by reduced feeding in both the oestradiol-treated and the control group. Treatment with oestradiol increased (P <0·01) basal plasma concentrations of IGF-I at both high and low levels of nutrition. After i.v. injection of bGH (0·1 mg/kg body weight), an increase in plasma IGF-I was observed only in steers at high nutrition. Basal concentrations of plasma IGF-II were not altered by nutritional manipulations but were significantly (P <0·001) increased by oestradiol treatment. After bGH infusion only steers at high nutrition showed an increase in plasma IGF-II. Significant correlations were observed between daily body weight gain and plasma concentrations of IGF-I (r= 0·91, P<0·001, n = 20) and also between the capacity of the high-affinity hepatic somatotrophic receptor and plasma IGF-I (r= 0·89, P <0·001, n= 10).

Decreased plasma concentrations of IGF-I at a low level of nutrition may abolish the growth-promoting activity of circulating GH. The increase in both GH secretion and the number of somatotrophic receptors with oestradiol treatment may represent a coordinated response of the somatotrophic axis leading to enhanced IGF-I and -II production and improved growth rate. The inferential relationships between the capacity of the high-affinity somatotrophic receptor, plasma concentrations of IGF-I and growth rates suggest that active modulation of somatotrophic receptors is an important regulatory constituent of the somatotrophic axis.

J. Endocr. (1988) 118, 243–250

ABSTRACT

The developmental pattern of plasma insulin-like growth factor-I (IGF-I) and insulin in calves subject to different patterns of weaning was investigated from birth until the age of 6 months. Fifteen male Friesian calves were fed on whole milk (10% of body weight per day) for the first 8 weeks after birth, then allocated into three balanced groups. Group 1 was weaned at 8 weeks; group 2 was weaned at 8 weeks, returned to milk-feeding at 13 weeks to be weaned again at the age of 16 weeks; group 3 was weaned at 12 weeks. After weaning the calves were fed on concentrates and lucerne hay.

At birth, circulating concentrations of IGF-I correlated with birth weight (r = 0·78, P< 0·001). There was a significant (P<0·001) fall in plasma IGF-I from birth (40·3 ± 2·5 μg/l) until 5 weeks (23·8± 1·3 μg/l), and then a gradual (P<0·01) rise until week 8 (35·0 ± 2·2 μg/l). Weaning (groups 1 and 2 after week 8) caused a significant (P<0·01) decrease in plasma IGF-I (20·5 ± 1·9 μg/l); thereafter plasma levels of IGF-I rose gradually (P<0·01) in animals fed on concentrates. The milk-fed calves (group 3) showed a progressive increase in plasma IGF-I with age until they were weaned at 12 weeks (51·0 ± 3·4 μg/l); IGF-I levels then decreased to be similar to group 1 (32·5 ± 2·1 μg/l). When group 2 was returned to milk-feeding, plasma IGF-I concentrations increased to 58·2 ±3·8 μg/l within 4 days and then continued to rise gradually until decreasing upon weaning. The age-related increase in the plasma concentration of IGF-I after 6 weeks was parallel in milk- and concentrate-fed calves.

Binding of ¹²⁵I-labelled bovine GH to hepatic membranes of neonatal calves was low at birth (specific binding; 1·56 ±0·29% n=3). Somatotrophic binding was apparent at 6 weeks (6·43 ±0·42%, n = 3) and increased (9·2± 1·1%, n=3) at the age of 12 weeks. Thus the early postnatal changes in plasma IGF-I may reflect the transition from GH-independent to GH-dependent IGF-I secretion.

Plasma concentrations of insulin at birth correlated with IGF-I (r = 0·54, P<0·05). However, regression analysis suggested no effect of insulin on birth weight independent of IGF-I. Plasma insulin levels rose gradually until weaning and thereafter were significantly (P<0·01) higher in the milk- than concentratefed calves.

Major changes in plasma concentrations of IGF-I related to monogastric (milk feeding) or ruminant (concentrate feeding) nutrition of the young calf are demonstrated. They may be mediated through changing plasma insulin concentrations. An age-dependent prepubertal rise in plasma IGF-I apparently independent of nutritional factors commenced with the appearance of functional somatotrophic receptors in the liver.

J. Endocr. (1988) 119, 43–50

Abstract

Plasma levels of IGFs-I and -II were measured in 4-month-old ewe lambs (n=20) and 2-year-old ewes (n=16), which were well fed (n=18) or fasted (n=18) for 3 days. Half of each nutrition group was given daily (0900 h) injections of bovine GH (bGH, 0·1 mg/kg body weight per day) for 3 days. Blood samples were collected immediately before the GH injection every morning.

Plasma IGFs were extracted by acid gel permeation chromatography using a Waters Protein Pak 125 column, fitted to a Pharmacia fast protein liquid chromatography system, then freeze-dried, reconstituted (at pH 7·4) and estimated by RIA.

At the end of the experiment, IGF-I levels in plasma were increased (P<0·01) by exogenous bGH in both fed ewes and lambs but not in the fasted animals; plasma IGF-I levels were depressed by fasting (P<0·01) at all ages. IGF-I levels were also found to be significantly higher (P<0·01) in ewes than lambs.

In contrast, plasma IGF-II concentrations were depressed (P=0·02) by administration of bGH in all groups and elevated in the ewes (P<0·05) by fasting. However, the lambs showed no significant changes in IGF-II with fasting. The IGF-II levels were significantly higher (P<0·001) in lambs than ewes.

Results from the present study demonstrate that GH administration stimulated an increase in plasma IGF-I and induced a decrease in plasma IGF-II. On the other hand, fasting depressed plasma IGF-I and elevated plasma IGF-II in the sheep. A significant GH/nutrition interaction for IGF-I (P<0·01), but not for IGF-II, and a significant nutrition/age interaction for IGF-II (P<0·01), but not for IGF-I, in the present study suggest that GH has a greater stimulating effect on plasma levels of IGF-I in the fed rather than fasted sheep and that nutrition has a greater influence on plasma levels of IGF-II in the older rather than younger animals, indicating that plasma IGFs-I and -II are differentially regulated by nutrition, GH and developmental stage in postnatal sheep.

Journal of Endocrinology (1995) 147, 507–516

ABSTRACT

The relationship between plasma GH profiles and circulating concentrations of insulin-like growth factor 1 (IGF-1) at three different planes of nutrition, chosen to represent a high, medium and low level of nutrition (3%, 1·8% and 1% dry matter of liveweight per day) was studied in 15 young Angus steers. All steers were maintained on 3% dry matter for 5 weeks, then on one of the three nutritional planes for 4 weeks and then all were returned to 3% dry matter for 3 weeks. Blood was sampled through jugular catheters at 15-min intervals for 25 h at the end of each phase of the study and additional samples were taken on 2 days each week.

Pulsatile release of GH occurred episodically with a diurnal increase during night and morning hours only in steers on high nutritional intakes. Reduced feeding at both the medium and the low plane abolished the diurnal rhythm and significantly increased mean plasma GH concentrations, the amplitude of GH pulses and the area under the GH profiles. Baseline concentrations of GH and pulse frequency did not change through nutritional manipulation. Upon realimentation, plasma GH concentrations decreased in both previously undernourished groups, with those fed 1% dry matter still having increased levels 10 days after refeeding. Plasma IGF-1 concentrations showed no periodicity. With nutritional deprivation, a decrease in IGF-1 concentration was observed only at negative energy balance (1% group). In this group plasma IGF-1 concentrations were progressively restored within 1 week of realimentation.

The different relationship between GH and IGF-1 release at each plane of nutrition suggests that at both medium and low levels of feed intake, tissue insensitivity to GH may exist peripherally and perhaps centrally. It is suggested that nutritional status may, through modulation of tissue sensitivity to GH, be a primary factor in determining growth and the regulation of the somatotrophic axis in the postnatal ruminant.

J. Endocr. (1986) 111, 209–215

Abstract

GH enhances skeletal muscle growth, and IGF-II peptide is highly expressed during regeneration. We have therefore investigated the effect of GH administration on IGF-II binding and expression in regenerating rat skeletal muscle using the techniques of receptor autoradiography and in situ hybridisation. Notexin, a myotoxin, was injected into the right M. biceps femoris (day 0), causing affected fibres to undergo necrosis followed by rapid regeneration. Animals were administered either GH (200 μg/100 g body weight) or saline vehicle daily. Contralateral muscles were used as regeneration controls. GH administration during regeneration resulted in significant increases in body weight, and damaged and undamaged muscle weights (P<0·001). IGF-II expression, which was examined in regenerating fibres, survivor fibres and undamaged fibres, varied according to tissue type (P< 0·001). Specifically, IGF-II expression in regenerating fibres was elevated relative to control and survivor fibres after day 3 (P<0·05), with a peak on day 9 (P<0·001). GH did not affect IGF-II message levels. ¹²⁵I-IGF-II binding in regenerating muscle was examined in the same fibre types as well as in connective tissue. ¹²⁵I-IGF-II binding in regenerating fibres was higher (P<0·001) than in other tissue types on day 5. GH administration increased ¹²⁵I-IGF-II binding in all damaged muscle tissues on day 5 (P<0·001, regenerating fibres; P<0·01, others). We believe that this shows for the first time an effect of GH on the Type 2 IGF receptor in regenerating skeletal muscle.

Journal of Endocrinology (1996) 149, 81–91

Abstract

The relative abundance and location of type 1 IGF receptors in sheep muscles have been measured to determine whether changes occur during post-natal growth and nutritional stress. Using the technique of histological autoradiography, specific binding of ¹²⁵I-IGF-I in muscle fibre and connective tissue of M. biceps femoris and M. gastrocnemius was demonstrated, as was specific binding to the tendon of M. gastrocnemius and the surrounding connective tissue. The binding site in both muscles was characterised as the type 1 IGF receptor in membrane preparations using competitive binding assay and SDS-PAGE.

Type 1 receptors were more abundant in connective tissue than muscle fibre or tendon (P≤0·001). Levels changed significantly with age in all tissues (P=0·054 to P≤ 0·001), while change as a result of fasting was limited to a receptor increase in the connective tissue of M. gastrocnemius (P=0·034). IGF-I mRNA in M. bicepsfemoris, as assessed by in situ hybridisation, showed changes in expression with increasing age (P≤ 0·025) but no change with fasting.

These data indicate that the distribution, relative abundance and nutritional sensitivity of type 1 receptors are related to cell type in vivo. The overall decline of receptors with increasing age may be a feature of transition from linear animal growth to cell maintenance in adult animals. Connective tissue appears to be more sensitive than muscle fibre to nutrition, possibly allowing the reduction of non-essential metabolism during fasting.

Journal of Endocrinology (1996) 148, 337–346